Garcia entered the round three shots behind overnight leader Rafa Cabrera-Bello but birdied six of his last 12 for a seven-under-par 65, forcing Ilonen to make an 18-foot birdie putt on the 18th hole to force the playoff with a 66. They both finished on 16-under 272.
Garcia missed an eight-foot birdie putt on the last that would have given him the win in regulation, and then couldn't make an eagle putt for the victory on the first playoff hole.
The Spaniard and Ilonen both had birdies on the second extra hole as well before Garcia sealed the victory with a tap-in birdie at the third to Ilonen's par.
“I started the day thinking if I can shoot a six or seven under, I can have a chance,” Garcia. “I'm very happy to be able to win it.”
Cabrera-Bello only managed a 69 and fell to a tie for third with Thorbjorn Olesen of Denmark (68).
Garcia's victory helped make up for last year's disappointment in Doha, when he finished runner-up after England's Chris Wood eagled the last hole to claim his first European Tour win. It also came on the heels of his victory at the Thailand Golf Championship in December, when he had his girlfriend Katharina Boehm caddie for him. Boehm was among the first to congratulate him after he sealed the win on Saturday.
“When things off the golf course are on good terms it is a little easier to concentrate when you don't have too many things in your head,” Garcia said. “I am very fortunate with Kathy and with my family and all those around me, so it's great to be playing well and to be in this winning situation with the goal now to keep improving.
“I still feel like I can do better and that is the goal.”
Choi leads at